Peter Olson <peabo@???> wrote:

> So I cleared out another partition and moved the backup of my Debian 8.3 onto
> it. Ran update-grub, which found the backup in its new location.
>
> But, when I try to boot it grub is confused and is pinned to the old UUID of the
> root filesystem. (I have already updated /etc/fstab in the restored backup, but
> it is not even getting that far.) It just dumps me into busybox saying it can't
> find the root fs.

From there, it should, if you know the magic incantation, be possible to manually mount the root fs etc and then continue. I don't know the magic incantation - perhaps ${pref_search_engine} will help.

But from a step back, at the grub menu, you can opt to edit (it's not persistent) the config - so you go and replace "root= ................" with "root=sdxn" which should allow it to boot.
Or, you can boot your normal system, list the UUID, write it down, and edit it into the grub config at boot time.

All this is why I much prefer filesystem labels (you can put "root=LABEL=oldroot" in the grub config to boot it) - and it's annoying that (at least in Debian) the only options are UIDs or device names.